M-ary orthogonal keying system

ABSTRACT

A digital modulation system provides enhanced multipath performance by using modified orthogonal codes with reduced autocorrelation sidelobes while maintaining the cross-correlation properties of the modified codes. For example, the modified orthogonal codes can reduce the autocorrelation level so as not to exceed one-half the length of the modified orthogonal code. In certain embodiments, an M-ary orthogonal keying (MOK) system is used which modifies orthogonal Walsh codes using a complementary code to improve the auto-correlation properties of the Walsh codes, thereby enhancing the multipath performance of the MOK system while maintaining the orthogonality and low cross-correlation characteristics of the Walsh codes.

This is a continuation of application Ser. No. 10/153,412, filed May 22,2002, which is a continuation of application Ser. No. 09/057,310, filedApr. 8, 1998, now issued as U.S. Pat. No. 6,404,732, which is acontinuation in part of application Ser. No. 08/688,574, filed Jul. 30,1996, and issued Jan. 19, 1999 as U.S. Pat. No. 5,862,182, thedisclosures thereof being incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to wireless communication systems and, moreparticularly, to a digital modulation system that uses modifiedorthogonal codes, such as M-ary orthogonal Keying (MOK) to encodeinformation.

2. Description of Related Art

A wireless communications channel can rarely be modeled as purelyline-of-site. Therefore, one must consider the many independent pathsthat are the result of scattering and reflection of a signal between themany objects that lie between and around the transmitting station andthe receiving station. The scattering and reflection of the signalcreates many different “copies” of the transmitted signal (“multipathsignals”) arriving at the receiving station with various amounts ofdelay, phase shift and attenuation. As a result, the received signal ismade up of the sum of many signals, each traveling over a separate path.Since these path lengths are not equal, the information carried over theradio link will experience a spread in delay as it travels between thetransmitting station and the receiving station. The amount of timedispersion between the earliest received copy of the transmitted signaland the latest arriving copy having a signal strength above a certainlevel is often referred to as delay spread. Delay spread can causeintersymbol interference (ISI). In addition to delay spread, the samemultipath environment causes severe local variations in the receivedsignal strength as the multipath signals are added constructively anddestructively at the receiving antenna. A multipath component is thecombination of multipath signals arriving at the receiver at nearly thesame delay. These variations in the amplitude of the multipathcomponents is generally referred to as Rayleigh fading, which can causelarge blocks of information to be lost.

Digital modulation techniques can be used to improve the wirelesscommunication link by providing greater noise immunity and robustness.In certain systems, the data to be transmitted over the wirelesscommunication link can be represented or encoded as a time sequence ofsymbols, where each symbol has M finite states, and each symbolrepresents N bits of information. Digital modulation involves choosing aparticular code symbol from the M finite code symbols based on the Nbits of information applied to the modulator. For M-ary keying schemes,log₂ M bits of information can be represented or encoded by M differentcodes or code symbols which are transmitted. The transmitted codes arereceived as several delayed replicas of the transmitted codes, and thereceiver correlates the delayed versions of the received codes with theknown codes by performing a summation of autocorrelation values for allpossible multipath delays.

The autocorrelation sidelobes show the correlation values between theknown codes and the time shifted replicas of the received codes. If acode is the same or is a shifted version of itself, then the code willhave a high level of autocorrelation or autocorrelation sidelobes. Forexample, for a code (111-1), the autocorrelation for a zero shift is:

code 1 1 1 −1 shifted code 1 1 1 −1 multiplication 1 1 1   1 correlation= sum of multiplied values = 4.For a shift of one chip, the autocorrelation is:

code 1 1   1 −1 shifted code 1 1   1 −1 multiplication 1 1 −1correlation = sum of multiplied values = 1.For a shift of 2 chips, the autocorrelation is:

code 1   1 1 −1 shifted code 1   1 1 −1 multiplication 1 −1 correlation= sum of multiplied values = 0.For a shift of 3 chips, the autocorrelation is:

code   1 1 1 −1 shifted code   1 1 1 −1 multiplication −1 correlation =sum of multiplied values = −1.Larger shifts give an autocorrelation value of zero, so the maximumautocorrelation sidelobe in this example has a value or magnitude of 1.In this example, −1's are used in the receiver instead of 0's. Theautocorrelation sidelobes give an indication about multipathperformance. If the autocorrelation sidelobes are large, severalmultipath components heavily interfere with each other.

Cross-correlation refers to a code being correlated with differentcodes. M-ary orthogonal keying is a form of digital modulation whichprovides good cross-correlation between codes by encoding data usingorthogonal codes which do not interfere with each other. FIG. 1 shows ageneral block diagram of an M-ary orthogonal keying system 10. In thisexample, input data is scrambled by a scrambler 12 as specified in thecurrent Institute of Electrical and Electronics Engineers (IEEE) 802.11standard. The data is then provided to a serial-to-parallel converter 14which converts the serial data into 8 parallel bits forming a datasymbol. A first modulator 16 receives three (3) of the parallel bits andproduces a code of length 8 chips from a look-up table, and a secondmodulator 18 receives three (3) of the parallel bits and produces asecond code of length 8 from a look-up table. Chips are actually codebits, but they are called chips to distinguish them from data bits. Inthis implementation, one of the parallel bits is provided to a firstexclusive-or (XOR) gate 20 which inverts the code from the firstmodulator if the bit has a value of one. Similarly, the last remainingbit is provided to a second XOR gate 22 which inverts the code from thesecond modulator 18 if the bit has a value of one. In this embodiment,the output I_(out) of the XOR gate 20 is applied to signal circuitry 21to convert all 0's to −1's for transmission. The circuitry 21 can alsomanipulate, convert and/or process I_(out) before being used to modulatea carrier with frequency ω by mixer 24. The output Q_(out) from the XOR22 is applied to signal circuitry 23 to convert all 0's into −1's fortransmission. The circuitry 23 can manipulate, convert and/or processQ_(out) before being used to modulate a 90 degrees shifted carrier bymixer 26. In this particular embodiment, the first modulator 16corresponds to the in-phase (I) component of the output signal, and thesecond modulator 18 corresponds to the quadrature (Q) component of theoutput signal.

The modulators 16 and 18 are performing M-ary orthogonal keying orencoding because each receive log₂ M bits of information and chooses oneout of M orthogonal codes. By having both I and Q components withdifferent polarities, a total of (2M)² possible code combinations exist,so a total of 2+2 log₂ M bits can be encoded into one orthogonal code.In this example, M is equal to 8. The M codes in an M-ary orthogonalkeying system are usually based on M chip Walsh codes. Using the M chipWalsh codes in an M-ary orthogonal keying system is advantageous becausethe M chip Walsh codes are orthogonal, which means they exhibit zerocross-correlation, so the M chip Walsh codes tend to be easilydistinguishable from each other. However, using Walsh codes as theorthogonal codes can create potential problems. For example, when Walshcode 0 (all 1's) is selected as the code symbol, Walsh code 0 may appearas an unmodulated continuous wave (CW) carrier signal.

To avoid the Walsh code 0 CW modulation, M-ary orthogonal keying systemshave been proposed which use a cover sequence of (11111100) to modifythe Walsh codes by inverting the last two bits of each Walsh code.Although the Walsh code 0 CW modulation is resolved by modifying theWalsh codes in this fashion, the modified Walsh codes retain the poorautocorrelation and spectral properties which are inherent to Walshcodes. To counter the poor autocorrelation and spectral properties ofthe Walsh codes, current systems multiply the output signal by apseudo-random noise (PN) sequence. Some systems multiply by a PNsequence having a length much larger than the Walsh code as described inE. G. Tiedemann, A. B. Salmasi and K. S. Gilhousen, “The Design AndDevelopment of a Code Division Multiple Access (CDMA) System forCellular and Personal Communications,” Proceedings of IEEE PIMRC,London, Sep. 23-25, 1991, pp. 131-136. Other systems multiply the Walshcodes by a PN sequence with the same length as the Walsh code. However,the autocorrelation properties of the resulting codes are still lacking.If the transmitted codes lack sufficient autocorrelation properties, themultipath performance of the system can be poor because the system willhave difficulty detecting the delayed or shifted versions of thetransmitted codes.

SUMMARY OF THE INVENTION

The present invention involves a digital (de)modulation system whichprovides enhanced multipath performance by using modified orthogonalcodes with reduced autocorrelation sidelobes while maintaining thecross-correlation properties of the modified codes. The modifiedorthogonal codes have autocorrelation sidelobes that do not exceedone-half the length of the modified orthogonal code. In certainembodiments, an M-ary orthogonal keying (MOK) system is used whichmodifies orthogonal Walsh codes using a complementary code to improvethe auto-correlation properties of the Walsh codes, thereby enhancingthe multipath performance of the MOK system while maintaining theorthogonality and low cross-correlation characteristics of the Walshcodes.

BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects and advantages of the present invention may becomeapparent upon reading the following detailed description and uponreference to the drawings in which:

FIG. 1 shows a block diagram of a M-ary orthogonal keying (MOK) systemusing Walsh codes modified by a cover sequence (11111100);

FIG. 2 shows a block diagram of a digital modulation system usingmodified orthogonal codes to reduce the autocorrelation sidelobes of theorthogonal codes;

FIG. 3 shows a block diagram of an embodiment of a MOK system accordingto the principles of the present invention;

FIG. 4 shows a graphical comparison of packet error ratio versus delayspread for a MOK system using Walsh codes modified by a cover sequencein current systems v. modified Walsh codes to reduce the autocorrelationsidelobes;

FIG. 5 shows a block diagram of another embodiment of the MOK systemaccording to the principles of the present invention;

FIG. 6 shows a graphical comparison of packet error ratio versus delayspread for embodiments of the MOK system using Walsh codes modified by acover sequence in current systems v. modified Walsh codes to reduce theautocorrelation sidelobes;

FIG. 7 shows a block diagram of another embodiment of the MOK systemaccording to certain principles of the present invention;

FIG. 8 shows a digital demodulator according to certain principles ofthe present invention;

FIG. 9 shows a demodulation system using the digital demodulatoraccording to certain principles of the present invention; and

FIG. 10 shows another embodiment of a demodulation system using thedigital demodulator according to the principles of the presentinvention.

DETAILED DESCRIPTION

Illustrative embodiments of the digital (de)modulation system to enhancemultipath performance for a wireless communications system is describedbelow. FIG. 2 shows a digital modulator 28 according to the principlesof the present invention. In response to data bits, the modulator 28chooses a corresponding one of M codes. The M codes are produced bymodifying a set of orthogonal codes to reduce the autocorrelation levelsassociated with those orthogonal codes while maintaining theorthogonality of the set. For example, if the same chip(s) in the codesof the orthogonal code set is inverted, the modified orthogonal codesremain orthogonal. In accordance with aspects of the present invention,an orthogonal code set is modified with another code to produce Morthogonal N-chip codes having autocorrelation sidelobes which do notexceed N/2 in value. The modulator 28 can perform the modification ofthe orthogonal codes using some processing circuitry implementing somelogic to perform the modification, or the modulator 28 can store themodified orthogonal codes in a look-up table. The modulator 28 can alsostore different sets of modified orthogonal codes depending on desiredchanges in operation or calculate different sets of the modifiedorthogonal codes. The modification of the orthogonal codes can beperformed by an element by element multiplication of the orthogonalcodes with a code having good autocorrelation properties. Thus, themodulator 28 produces codes with low autocorrelation properties whilemaintaining at least some of the orthogonality characteristics of theoriginal orthogonal codes. In this embodiment, the data bits are shownas being received in parallel, and the code chips are shown as beingproduced serially. Depending on the application, the data bits can bereceived serially, and/or the code chips can be produced in parallel.

Complementary codes or sequences are sets of sequences characterized bythe property that for shifts in the sequences the autocorrelations ofthe sequences sum to zero except for the main peak at zero shift. Assuch, complementary codes can be used to modify the set(s) of orthogonalcodes of the modulator 28. Complementary codes are discussed in RobertL. Frank, “Polyphase Complementary Codes.” IEEE Transactions OnInformation Theory, Vol. IT-26, No. 6, November 1980, pp. 641-647. Forlengths equal to a power of two, complementary codes are easilygenerated by the following rule; starting with sequences A=B={1}, acomplementary code of twice the length is given by ABAB′, where B′ meansinverting all elements of sequence B. Hence, for lengths 2 up to 16,complementary sequences are:

{1 0} {1 1 1 0} {1 1 1 0 1 1 0 1} {1 1 1 0 1 1 0 1 1 1 1 0 0 0 1 0}Additionally, other transformations can be done on a complementary codeto generate other complementary codes from the same length. Forinstance, it is possible to reverse the first or second half of thecode, so {1 1 1 0 1 0 1 1} would be another complementary code of length8.

Complementary codes have low auto-correlation sidelobes, and acomplementary code multiplied by a Walsh function produces anothercomplementary code. As such, if a complementary code is used to modify aWalsh code set, the resulting modified Walsh codes are complementary andhave the same low auto-correlation sidelobes. The modified Walsh codeset also remains orthogonal, which means that the cross correlationbetween any two different codes is zero (for a zero delay).

FIG. 3 shows an embodiment of a MOK system 30 using modulators 32 and 34to produce length 8 codes in response to 3 information bits from theserial to parallel converter 14. In this embodiment, the set oforthogonal codes is the length 8 Walsh code set, and the Walsh code setis modified using a complementary code. The length 8 Walsh code set is:

1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 1 1 0 0 1 1 0 0 1 0 0 1 1 0 0 1 1 0 1 01 0 1 0 1 0 1 0 0 1 0 1 1 0 0 1 0 1 1 0 1 1 0 0 0 0 1 1In previous systems, the Walsh codes are modified by an element byelement exclusive-or with the code {1 1 1 1 1 1 0 0}, so the last twochips of each Walsh code (or the chips of the last two columns of theWalsh code set) are inverted. This modifying code, however, hasauto-correlation sidelobes with a worst-case magnitude of 5 (using −1'sfor the 0's), which is an autocorrelation value greater than one-halfthe length of the 8 chip code and produces multipath performanceproblems.

Instead, in the embodiment of FIG. 3, the MOK system 30 uses (a) length8 complementary code(s), for example the sequences {1 1 1 0 1 1 0 1} or{1 1 1 0 1 0 1 1}, to modify the length 8 Walsh code set. For the lattercode, the modified Walsh code set appears as:

1 1 1 0 1 0 1 1 1 1 1 0 0 1 0 0 1 1 0 1 1 0 0 0 1 0 0 0 1 1 0 1 1 0 1 11 1 1 0 1 0 1 1 0 0 0 1 1 0 0 0 0 0 1 0 1 1 0 1 0 1 1 1This modified Walsh code set produces worst-case autocorrelationsidelobes having a magnitude or value of only 2. As such, this modifiedcode compares favorably in performance to complementary Barker codeswhich have autocorrelation sidelobes bounded to only one. ComplementaryBarker codes are discussed in Robert L. Frank, “Polyphase ComplementaryCodes.” IEEE Transactions On Information Theory, Vol. IT-26, No. 6,November 1980, pp. 641-647. However, Barker codes or sequences onlyexist for certain odd lengths such as length 11. The first of the twocomplementary codes specifically mentioned above has improvedcross-correlation properties for time shifted codes.

In the operation of the embodiment of FIG. 3, the scrambler 12 receivesdata and scrambles the data according to the IEEE 802.11 standard. Inother embodiments, the scrambler 12 may not be necessary, and the datacan be manipulated by some other form of data conversion, interleavingor modification, or the data can be fed directly into theserial-to-parallel converter 14. In this embodiment, theserial-to-parallel converter 14 is a 1:8 multiplexer (MUX) whichproduces a data symbol of 8 data bits in parallel according to a 1.375MHz clock signal. The eight bit data symbol is encoded into a symbolcomprising a I/Q code pair of 8 chip codes or codewords, so the symbolinterval is equal to the code length. Three (3) of the bits of the datasymbol are provided to the first modulator 32 which produces acorresponding length 8 Walsh code which has been modified by acomplementary code. The first modulator 32 produces the length 8 Walshcode at a chip rate of about 11 MHz as dictated by an 11 MHz clocksignal. In the above example, each symbol contains 8 data bits, whichare encoded into independent I and Q codes of 8 chips. Chips areactually code bits, but they are called chips to distinguish them fromdata bits. In this embodiment, the first modulator 32 corresponds to theI phase modulation branch of the MOK system 30 which produces the Icomponent of the of the signal to be transmitted.

A second set of three (3) bits of the data symbol from the converter 14is provided to the second modulator 34 which produces a correspondinglength 8 Walsh code which has been modified using a complementary code.The second modulator 34 corresponds to the Q phase modulation branch ofthe MOK system 30 which produces the Q component of the of the signal tobe transmitted. In response to the three data bits, the second modulator34 also produces a length 8 Walsh code at a chip rate of about 11 MHz asdictated by the 11 MHz clock signal.

Of the remaining two of eight bits of the data symbol from the serial toparallel converter 14, one is provided to a first XOR gate 36. If thebit is a 0, the first XOR gate 36 changes the polarity of the length 8Walsh code from the first modulator 32. The resulting modified Walshcode I_(out) is provided to signal circuitry 21 to change any 0's to 1'sand perform any additional signal processing and/or conversion beforebeing provided to the first mixer 24 to modulate a carrier of frequencyω. The last remaining bit is provided to a second XOR gate 38. If thebit is a 0, the second XOR gate 38 changes the polarity of the length 8Walsh code from the second modulator 34. The resulting modified Walshcode Q_(out) is provided to the signal circuitry 23 for any conversionand/or processing before being provided to the second mixer 26 tomodulate a 90 degree shifted version of the carrier with frequency ω. Ifinstead of 0's, −1's are used, the first and second XOR gates 36 and 38,can be replaced by multipliers to change the polarity of I_(out) andQ_(out). Subsequently, the I_(out) modulated carrier and the Q_(out)modulated carrier are combined and transmitted. As such, this particularembodiment of the MOK system 30 partitions 8 bits of incoming data into4 bits for the I branch and 4 bits for the Q branch. The three data bitson the I branch are encoded into a code of 8 chips, and the three databits on the Q branch are encoded in parallel into a code of 8 chips.Because the last two bits encode information by determining the polarityof the 8 bit symbols respectively, the MOK system 30 encodes 8 data bitsinto 2 codes which are both picked from a set of 16 possible codes. Inthis example, there are 8 modified Walsh codes, which can be inverted toget 16 codes. With a symbol rate of 1.375 MSps and 8 bits/symbol, thedata rate for the MOK system 30 is 11 MBps.

FIG. 4 shows a graphical comparison of the packet error ratio v. delayspread (ns) in multipath fading channels using 8 bits per symbol at 11Mbps and a 4 taps channel matched filter as would be understood by oneof ordinary skill in the art. Curve 40 corresponds to digital modulationusing Walsh codes modified by the cover sequence (11111100) of systemsof the current art, and curve 42 corresponds to digital modulation usingWalsh codes modified by a complementary code (11101011) according to theprinciples of the present invention. The channel model used has anexponentially decaying power delay profile and independent Rayleighfading paths. FIG. 4 shows that by using the complementary code, thesystem can tolerate a delay spread that is about 50% larger (curve 42)than for the other code (curve 40) to achieve a packet error ratio of 1%or 10%.

FIG. 5 shows an embodiment of a MOK system 50 which can be used as afallback mode for the MOK system 30 (FIG. 3). Once again, the input datais scrambled by the scrambler 12 according to the IEEE 802.11 standard.The data is provided to a serial to parallel converter 52. The serial toparallel converter 52 in this embodiment produces 5 bit data symbols inparallel at a data symbol rate of 1.375 MSps. From the 5 bit datasymbol, three bits are received by a modulator 54 which encodes the 3bits into a length 8 modified Walsh code according to the principles ofthe present invention. The length 8 modified Walsh code is provided toboth I and Q branches 56 and 58. In accordance with another inventiveaspect of this particular embodiment, by providing the same code tomultiple phase modulation paths or branches, this embodiment allows afallback mode with independent phase modulation, such as quadraturephase shift keying (QPSK) or 8-phase shift keying (8-PSK), of the samecode on the multiple phase modulation paths, such as the I and Qbranches 56 and 58 in this embodiment. On the I branch 56, the 8 chipmodified Walsh code is serially provided to a first XOR gate 60, and onthe Q branch 58, the 8 chip Walsh code is serially provided to a secondXOR gate 62. Of the two remaining bits from the serial to parallelconverter 52, one bit goes to the first XOR gate 60 to adjust thepolarity of the length 8 modified Walsh code and produce I_(out) on theI branch 56, and the other bit goes to the second XOR gate 62 to adjustthe polarity of the length 8 modified Walsh code and produce Q_(out) onthe Q branch 58. Depending on the implementation, if −1's are usedinstead of 0's, the first and second XOR gates 60 and 62 can be replacedby multipliers. As such, given data symbols of 5 bits/symbol and asymbol rate of 1.375 MBps, this embodiment provides a data rate of 6.8MBps.

FIG. 6 shows a graphical comparison of the packet error ratio v. delayspread (ns) in multipath fading channels using 1) Walsh codes modifiedby the cover sequence (11111100) of current systems with quadraturephase shift keying (QPSK) at a fallback rate of 6.8 Mbps (curve 63), 2)Walsh codes modified by a complementary code (for example, 11101011)using 8-phase shift keying (8-PSK) at 8.25 Mbps (curve 64), and 3) Walshcodes modified by a complementary code (for example, 11101011) usingQPSK at a fallback rate of 6.8 Mbps and the same code on I and Qbranches (curve 65). The channel model used has an exponentiallydecaying power delay profile and independent Rayleigh fading paths. FIG.6 shows that the delay spread tolerance is more than doubled by usingthe codes proposed by the invention. Additionally, FIG. 6 shows that thedigital modulation system can be used with alternative modulationschemes, such as 8-PSK instead of QPSK, to get a higher data rate (8.25Mbps) without losing much delay spread performance as would beunderstood by one of skill in the art.

FIG. 7 shows an embodiment of a MOK system 66 which can be used as afallback mode for the MOK system 30 (FIG. 3). The input data isscrambled by the scrambler 12 according to the IEEE 802.11 standard. Thescrambled data is provided to a serial to parallel converter 68. Theserial to parallel converter 68 in this embodiment produces 4 bit datasymbols in parallel at a symbol rate of 1.375 MSps. From the 4 bit datasymbol, three bits are received by a modulator 70 which encodes the 3bits into a length 8 modified Walsh code according to the principles ofthe present invention. The modulator 70 serially produces the length 8Walsh code at a rate of 11 MHz. The length 8 modified Walsh code isprovided to an XOR gate 72 corresponding to both the I and Q branches.The length 8 modified Walsh code is multiplied by the remaining bit ofthe data symbol from the serial-to-parallel converter 68 to adjust thepolarity of the length 8 code and produce I_(out) and Q_(out) in serialform. Depending on the implementation, if −1's are used instead of 0's,the XOR gate 72 can be replaced by a multiplier. As such, given datasymbols of 4 bits/symbol and a symbol rate of 1.375 MBps, thisembodiment provides a data rate of 5.5 MBps.

FIG. 8 shows a digital demodulation system 76 which can be used at areceiver (not shown) to receive transmitted codes from a transmitter(not shown) using an embodiment of the digital modulation systemdescribed above. The digital demodulation system 76 receives a modifiedorthogonal code according to the principles of the present invention. Inresponse to the modified orthogonal code, the digital demodulationsystem produces a corresponding data symbol. Depending on the particularimplementation, the code chips and/or the data bits can be in parallelor in series.

FIG. 9 shows a demodulation system 80 using the digital demodulationsystem according to the principles of the present invention. In thisparticular embodiment, the received signal is supplied to both I and Qbranches 82 and 84 of the demodulation system 80. A first mixer 86multiplies the received signal by the cos ωt, where ω is the carrierfrequency, to extract the modulated I information, and a second mixer 88multiplies the received signal by sin ωt to extract the modulated Qinformation. After low pass filtering, the I and Q information areprovided to correlator blocks 90 and 92, respectively. In thisparticular embodiment, the correlator blocks 90 and 92 contain 8correlators for correlating time delayed versions of the I informationand the Q information, respectively. The find code blocks 94 and 96 findthe known modified orthogonal codes according to the present inventionwhich give the highest correlation magnitudes for the I and Qinformation. In certain embodiments, the demodulator 76 (FIG. 8) orportions thereof can be performed in or receive the output from the findcode blocks 94 and 96 to decode the known orthogonal codes intocorresponding data bits. Depending on the embodiment, the digitaldemodulation system 76 (FIG. 8) or portions thereof can be implementedin the find code blocks 94 and 96, in the detect polarity blocks 98 and100, branching off of the of the I and Q paths 82 and 84 and/or at theoutput of detect polarity blocks 98 and 100 to decode the modifiedorthogonal codes to produce the corresponding data bits. In thisembodiment, the detect polarity blocks 98 and 100 each decodes anadditional data bit each from the polarity of the found modifiedorthogonal code.

FIG. 10 shows an embodiment of a demodulation system 110 which can beused at the fallback rate for the demodulator system 80 (FIG. 9)receiving code symbols from the modulator system 50 (FIG. 5) where thesame code is transmitted on multiple modulation paths. The differencebetween the demodulation system 110 and the full rate demodulationsystem of FIG. 9 is that the code detection block 112 adds the squaredcorrelation outputs of the I and Q correlators 90 and 92 and detects themodified orthogonal code according to the present invention which gavethe highest correlation complex magnitude. In accordance with aninventive aspect of this particular embodiment, the same code is on boththe I and Q paths 82 and 84 for digital demodulation. In this particularembodiment, a block 114 finds the modified orthogonal code with thehighest complex correlation magnitude. In certain embodiments, thedemodulator 76 or portions thereof can be performed in or receive theoutput from the find code block 112 to decode the modified orthogonalcodes into corresponding data bits. Depending on the embodiment, thedigital demodulation system 76 (FIG. 8) or portions thereof can beimplemented in the code detection block 112, in a phase detector 114,branching off of the path 116 and/or at the output of the phase detector114 to decode the modified orthogonal codes and produce thecorresponding data bits. The phase detector 114 detects the phase of thecomplex correlation output to decode an extra 2 bits per code symbol forQPSK or an extra 3 bits per code symbol for 8-PSK.

In addition to the embodiment described above, alternativeconfigurations of the digital (de)modulation system according to theprinciples of the present invention are possible which omit and/or addcomponents and/or use variations or portions of the described system.For example, the above applications use a Quadrature Phase Shift Keying(QPSK) phase shift modulation scheme (FIG. 1, 3, 5) along with thedigital modulation scheme and a binary phase shift keying (BPSK) scheme(FIG. 6), but the digital modulation system can be used with othermodulation schemes, such as amplitude modulation including quadratureamplitude modulation (QAM) and other phase modulation schemes including8-phase shift keying (8-PSK) as would be understood by one of ordinaryskill in the art. Additionally, The digital modulation system has beendescribed as using orthogonal codes of 1's and 0's which are modified bycodes of 1's and 0's, but the digital modulation system can be performedusing codes of 1's and −1's or 1's and 0's depending on the embodiment.In the embodiments described above, codes of 1's and −1's are receivedat the receiver, and the correlation determinations are described interms of 1's and −1's, but the demodulation system can use 1's and 0'sor 1's and −1's depending on the embodiment.

Furthermore, the digital modulation system has been described using aparticular configuration of distinct components, but the digitalmodulation system can be performed in different configurations and inconjunction with other processes. Additionally, the various componentsmaking up the digital modulation system and their respective operatingparameters and characteristics should be properly matched up with theoperating environment to provide proper operation. It should also beunderstood that the digital modulation system and portions thereof canbe implemented in application specific integrated circuits,software-driven processing circuitry, firmware, lookup-tables or otherarrangements of discrete components as would be understood by one ofordinary skill in the art with the benefit of this disclosure. What hasbeen described is merely illustrative of the application of theprinciples of the present invention. Those skilled in the art willreadily recognize that these and various other modifications,arrangements and methods can be made to the present invention withoutstrictly following the exemplary applications illustrated and describedherein and without departing from the spirit and scope of the presentinvention.

1. A method of processing information bits, said method comprising thesteps of: producing, by a modulator, in response to a set of saidinformation bits, a modified code which was modified from an orthogonalcode, using a complementary code, to provide autocorrelation sidelobesfor said modified code which are equal to or less than one-half thelength of said modified code, wherein said orthogonal code is a Walshcode.
 2. The method of claim 1 wherein said step of producing includesthe step of: responding to a set of log₂M bits to produce said modifiedcode as one of M modified codes.
 3. The method of claim 2 wherein saidstep of producing includes the step of: producing said modified codehaving a length of M.
 4. The method of claim 3 further including thestep of: using M=8.
 5. The method of claim 1 further comprising the stepof: providing said modified code to multiple modulation paths formodulation of said modified code on said modulation paths.
 6. A methodof processing information bits, said method comprising the steps of:producing, by a digital modulation system, in response to a set of saidinformation bits, a modified Walsh code which was modified from a Walshcode using a complementary code.
 7. A digital modulation system, saidsystem comprising: a modulator responsive to a set of information bitsto produce a modified code, said modified code being modified from anorthogonal code, using a complementary code, to provide autocorrelationsidelobes for said modified code which are equal to or less thanone-half the length of said orthogonal code, wherein said orthogonalcode is a Walsh code.